Synthesis of Iridium Oxide Nanotubes by Electrodeposition into ...

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Synthesis of Iridium Oxide Nanotubes by Electrodeposition into Polycarbonate Template: Fabrication of Chromium(III) and Arsenic(III) Electrochemical Sensor Erfan Mafakheri,a, c Abdollah Salimi,*a, b Rahman Hallaj,a Abdolali Ramazani,c Mohamad Almasi Kashic a

Department of Chemistry, University of Kurdistan, P. O. Box 416, Sanandaj-Iran tel: + 98-871-6624001, fax: + 98-871-6624008 b Research Center for Nanotechnology, University of Kurdistan, P. O. Box 416, Sanandaj-Iran c Department of Physics, University of Kashan, Kashan 87317-51167, Iran *e-mail: [email protected]; [email protected] Received: June 24, 2011;& Accepted: July 22, 2011 Abstract For the first time iridium oxide (IrO2) nanotubes are synthesized by electrodeposition in a polycarbonate (PC) template. Potential cycling (90 cycles) between 0.0 and 0.9 V is used for the preparation of IrOx nanotubes onto the PC template with a pore diameter of 100 nm. Field-emission scanning electron microscopy (FESEM) images show, that IrO2 nanotubes with uniform diameters of 110 10 nm and an estimated length of 1–3 mm are formed. The electrochemical properties and the electrocatalytic activity of a glassy carbon-IrOx nanotube modified electrode toward Cr3 + and As3 + oxidation are investigated. Finally, the modified electrode is used for micromolar detection of the proposed analytes using differential pulse voltammetry. Keywords: Iridium oxide, Nanotubes, Glassy carbon, Electrodeposition, Cr(III), As(III), Sensors

DOI: 10.1002/elan.201100332

1 Introduction One dimensional nanoscaled materials such as nanorods, nanowires and nanotubes, proved to be an ideal system to study the effect of low dimensionality on their physical and chemical properties. The large surface area of these one-dimensional nanostructures plays an important role in governing their properties [1]. Among different nanomaterials tubular nanostructures (nanotubes) have stimulated extensive research efforts in recent years because of their technological importance in advanced electronic devices, and prospective application in sensors and biosensors, biological separations, transport and energy storage/ conversion [2–6]. Therefore, various methods including, hydrothermal pyrolysis [7], atomic layer deposition [8], chemical vapor deposition [9], electroplating [10], electroless plating [3], sol-gel and pyrolyzing [11], supercritical fluids [12] and template based growth [6, 13, 14] have been developed for fabrication of tubular nanostructures. Among these methods, template based synthesis has been a simple and versatile approach employed to fabricate nanotubes, which entails synthesizing the desired materials within pores of a nanoporous template membrane. Then, formation of nano- and microporous templates with a high order in their geometry and uniform structure in pores is an exciting example for efforts toward making different type of templates. Different kinds of template Electroanalysis 2011, 23, No. 10, 2429 – 2437

including polycarbonate(PC) membranes [3, 11, 15, 16], mesoporous silica [17], micelle [18] and porous anodized aluminum oxide (AAO) [2, 6, 19] have been employed. Pore diameters of 30–400 nm can be achieved for AAO and 2–20 nm for sol-gel films, but there is still low uniformity and scalability. Inversely, porous templates made of zeolites have very uniform pores, but in a very narrow range of 0.3–3 nm [20]. The pores in AAO have a length of tens of mm and cannot be simply shortened because of the chemical mechanisms that produce nanopores. In addition the nearby nanostructures with a large aspect ratio may easily aggregate and lodge when the AAO template is completely removed. This can have a great influence on the electron transport properties of nanostructures, thus limiting otherwise promising applications of nanoelectronics [21]. Therefore, in comparison to AAO, PC membranes are suitable templates for preparation of nanostructures. By using the template method different properties such as shape, size and composition of nanotubes can be controlled easily; also a high aspect ratio can be achieved [15]. Template synthesis is especially useful to produce nanotubes because the cylindrical wall of the nanopores can be a starting point of chemical reactions. This is today the easiest way to form a tubular structure whereby nearly any material can be integrated into nanotubes. In recent years nanotubes of different metals [3, 6, 22], metal oxides

2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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[11, 23, 24], semiconductors [25, 26] and conducting polymers [16, 27] have been synthesized in porous membranes by electrochemical deposition, chemical vapor deposition, chemical polymerization and electroless deposition. The electrochemical synthesis in template membranes has been taken as one of the most efficient methods in controlling the growth of nanostructures (nanotubes and nanowires) because their growths are controllable almost exclusively in the direction normal to the substrate surface [25]. In electrodeposition technique for fabrication of nanotubes, the properties of the electrodeposits are determined by many factors including the electrolyte composition, pH, temperature and agitation, the applied electrode potential and/or the current density. The electrochemical deposition in polycarbonate membranes facilitates the control of deposition parameters such as temperature, applied voltage and current density and provides the highest flexibility compared to other techniques. Metal oxide nanostructures are suitable matrixes and novel candidates for fabrication of sensing and biosensing devices due to their high electrical conductivity, wide electrochemical working window, high biocompatibility, large surface area, no toxicity, chemical and photochemical stability, electrochemical activity, ease of preparation and excellent substrate adhesion [27]. Due to important applications of metal oxides significant efforts for the synthesis of certain metal oxide nanotubes such as, IrO2 [28], VOx [29], TiO2 [30], ZnO [31], ZrO2 [32], Al2O3 [33], In2O3 [34] and W18O49 [35] have been brought out. IrO2 is a conductive material and shows chemical and thermal stability. It was used as important and suitable matter for fabrication of electrodes in electrochemical systems as well as construction of microelectronic devices [36–38]. Furthermore, it can be used as supercapacitor, electrochromic display device, in water treatment, clinical diagnostics devices, power sources and for oxygen reduction [39–44]. In addition, IrO2 has also been studied as a high performance field emitter through its low work function, low resistivity and excellent stability against oxygen [45, 46]. Due to reversible faradic reactions for the Ir3 + /Ir4 + redox couple in a wide pH range, the IrO2 based modified electrodes have been used for pH sensing, electrocatalytic reactions and electroanalysis [47–54].The synthesis of IrO2 nanorods and their practical applications has been reported [55–57]. To the best of our knowledge, there is only one report on preparation of IrO2 nanotubes [28]. The IrO2 nanotubes have been grown on LiTaO3 substrates via metal organic chemical vapor deposition, using specific reagent (methylcyclopentadienyl) (1,5-cyclooctadiene) iridium (I) as the source [58]. In the present study a simple electrodeposition technique is used for preparation of, IrO2 nanotubes, using polycarbonate templates and K3Ir(Cl)6 as the reagent source. The surface morphology and electrochemical properties of the prepared nanotubes were examined with field emission scanning electron microscopy (FESEM) and cyclic voltammetry. The electrocatalytic activity of the IrOx-nanotube modified glassy carbon elec2430

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trode toward oxidation of As(III) and Cr(III) were investigated. Differential pulse voltammetry was used for micromolar detection of the selected analytes.

2 Experimental 2.1 Chemicals and Apparatures Potassium hexachloroiridate (K3Ir(Cl)6) was obtained from Aldrich. Polycarbonate membranes with a pore diameter of 100 nm and pore densities of 109 (pores/cm2) was obtained from Whatman. All other chemical materials used in the experiments were obtained from Merck. Aqueous solutions were prepared using doubly distilled deionized water and deposition solutions were deoxygenated by purging with highly purified argon gas. All electrochemical measurements are carried out with a threeelectrode system comprising modified and unmodified glassy carbon as a working electrode, a Ag/AgCl (3 M KCl), as a reference electrode and a platinum wire as an auxiliary electrode. Cyclic and differential pulse voltammograms are performed with a computer controlled mAutolab modular electrochemical system (Eco Chemie Ultecht, The Netherlands), driven with GPES software (Eco Chemie). All measurements are performed at room temperature (25 8C). Electrodeposition of IrOx nanotubes was done in a cylindrical teflon cell, with the substrate (membrane) facing upward against a hole on the bottom of the cell. A Pt counter electrode, facing up the membrane electrode, and an Ag/AgCl (3 M KCl) as a reference electrode were used. The following experimental parameters were used to record the differential pulse voltammograms: pulse amplitude 50 mV; pulse time 40 ms; and scan rate 10 mV s1. 2.2 Synthesis of Iridium Oxide Nanotubes The deposition solution of iridium was prepared pursuant to a two-step procedure carried out by Baur [58]. The first step is the formation of the diaquotetrachloroiridate(III) ion, Ir(H2O)2Cl4 , from Na3Ir(Cl)6. A 7.5 mM solution of Na3Ir(Cl)6 in 0.1 M HCl was dissolved by heating at 80 8C for 2 h. The second step in the preparation of the deposition solution was the formation of iridium(III) oxide from Ir(H2O)2Cl4 with added base according to the following reaction: IrðH2 OÞ2 Cl4 þ NaOH ! Ir2 O3 xH2 O

ð1Þ

Prior to the addition of base, oxygen must be removed because the iridium(III) oxide is unstable in the presence of oxygen. After removing the oxygen from the acidic solution of Ir(H2O)2Cl4 , the pH of the solution was raised to 10.5 by adding anhydrous potassium carbonate. This solution was used for the deposition of iridium oxide into the polycarbonate templates. Prior to IrOx electrodeposition, a thin gold film was sputtered onto one side of the polycarbonate template in order to make it conductive


Electroanalysis 2011, 23, No. 10, 2429 – 2437

Synthesis of Iridium Oxide Nanotubes by Electrodeposition into Polycarbonate Template

and to serve as the working electrode. The gold-coated polycarbonate templates were attached onto the glassy carbon electrode using carbon glue. Cyclic voltammetry is used as simple and fast method for electrodeposition of IrOx nanotubes into the polycarbonate template. For this purpose, the potential was scanned between 0.0 and 0.9 V vs. reference electrode at 50 mV s1 for 90 cycles in 0.1 M deposition solution. The PC membranes were dissolved with dichlorometane for SEM study. In order to use in electrochemical studies the PC template was removed partially by rinsing with 50/50 (v/v) of CH2Cl2 and EtOH.

3 Results and Discussions 3.1 Electrochemical Behavior and Surface Morphology of IrOx Nanotubes Cyclic voltammetry is used as simple and fast method for electrodeposition of IrOx nanotubes into the polycarbonate template. Figure 1 shows cyclic voltammograms resulting from continuous potential cycling of the electrode in the range from 0.0 to 0.9 V (90 cycles). The current on the consequence scans increases indicating formation of an electroactive deposit on the electrode surface. The formation of IrO2 is known to be initiated electrochemically at the surface PC templates modified GC electrode based on the following reaction [58].

Fig. 1. Continuous potential cycling (90 cycles) at a GC/PC template in a plating solution containing 7.5 mM of Na3Ir(Cl)6 (pH 10.5), scan rate 100 mV s1 . Electroanalysis 2011, 23, No. 10, 2429 – 2437



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E. Mafakheri et al.

ð2Þ

The formation of Iridium oxide nanostructures on the polycarbonate template was checked by recording cyclic voltammograms of the modified electrode in 0.1 M phosphate buffer solution (pH 5) free of iridium ions at a scan rate of 20 mV s1 (Figure 2). As shown well-defined redox couples were observed for the modified electrode, indicating that the electrodeposition method is a successful technique for electrodeposition of iridium oxide nanostructures onto the nanopores of the PC template membrane. Figure 3 shows the recorded cyclic voltammograms of the modified electrode at different scan rates. As shown in the inset of Figure 3 the peak currents linearly increased with increasing scan rate as expected for a redox surface controlled process. Based on the Laviron theory [59] the electron transfer rate constant (ks) and charge transfer coefficient (a) can be determined by measuring the variation of the peak potential with scan rate. A charge transfer coefficient, a = 0.40 was obtained using the equation Ep = K2.3030 (RT/nF) log(v) and one electron was transferred for Mn-complex. Using this value in the following equation [59]:

log ks ¼ a logð1aÞ þ ð1aÞ log a logðRT=nFvÞað1aÞ nF DE=2:3RT

ð3Þ

an apparent electron transfer rate constant, ks = 3.38 s1, was estimated for IrOx nanotube modified GC electrode. Figure 4 shows the FE-SEM image of the template and electrodeposited iridium oxide nanotubes from top view before solving the membrane. As can be seen there is no specific order because of random propagation of the pores. In Figure 4b the magnification is higher and nanotube like constructions are observed. Based on the SEM image the nanotube diameter and their density is about 1.05 109 cm2 corresponding to the pore density of the PC template. Furthermore, the outer diameter of the nanotube is in the range of 100–110 nm, which corresponds to the pore diameter of PC membrane. Increasing the cycle numbers (n 120 cycles ) nanotubes would gradually be exposed from the PC template and IrOx congregate to many clusters (Figure 5). Therefore 90 potential cycles were used as optimum cycle number for preparation of IrOx nanotubes. The morphology of the nanotube array was characterized by SEM after dissolving of PC template. The SEM images of the prepared nanotubes confirmed the formation of IrOx

Fig. 2. Cyclic voltammograms of bare (a) and iridium oxide nanotube modified electrode (b) in pH 5 buffer solution; scan rate of 20 mV s1.

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Fig. 3. Cyclic voltammetric responses of IrO2 nanotube modified electrode in pH 5 buffer solution at different scan rates; (inner to outer) 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV s1. Inset: plot of peak currents vs. scan rate.

nanotubes (Figure 6). As illustrated the IrOx nanotubes are uniform, with an average diameter of about 110 10 nm which corresponds to the size of the nanopores in the template. Furthermore, the estimated length of the nanotubes is about 1–3 mm. 3.2 Electrocatalytic Oxidation of Chromium(III) and Arsenic(III) at IrOx-Nanotube Modified Electrode Trivalent chromium (Cr3 + ) is a nutritional component for a large class of organisms [60]. Trace amounts of Cr3 + influences sugar and lipid metabolism in humans and its deficiency is suspected to cause a disease called chromium deficiency [61]. Arsenic compounds, especially their inorganic derivatives are known to be toxic and it has harmful effects on animals and the ecosystem [62]. Furthermore, the toxicity of arsenic is greatly dependent on the amount of As3 + , since As3 + is 50 times more toxic than arsenate due to its reactions with enzymes in human metabolism [63]. There are many reports in the literature about the higher risks of skin, bladder, lung, liver and kidney Electroanalysis 2011, 23, No. 10, 2429 – 2437

Fig. 4. FESEM imags of top view of electrodeposited iridium oxide nanotubes in PC membrane (a); magnified image of iridium oxide nanotubes (b); the cycle numbers for electrodeposition is 90 cycles; (c) FESEM image of PC template.

cancer, as well as other skin diseases such as hyperkeratosis and pigmentation changes that result from continued consumption of elevated levels of arsenic in drinking water [64, 65]. Therefore, it is clear that a simple, easy to handle and inexpensive method to analyze of As(III) and Cr(III) with sufficient sensitivity is required. Owing to the low costs, easy operation and high sensitivity, electrocatalysis through different redox mediation is an attractive technique for routine Cr3 + and As3 + anal-



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Fig. 5. FESEM image of top view of electrodeposited iridium oxide nanotubes in PC membrane, the cycle numbers for electrodeposition is 120 cycles.

Fig. 7. Recorded cyclic voltammograms of GC (a), GC in the presence 80 mM of Cr3 + (b), (c and d) as (a and b) for IrO2 nanotube modified electrode in phosphate buffer solution (pH 5) at scan rate 20 mV s1.

Fig. 6. FESEM image of prepared iridium oxide nanotubes after disolving of PC template.

ysis. Because of electrochemical reversibility and high electron transfer rate constant of the Ir(III)/Ir(IV) redox couple at wide pH range, it can be used as mediator for 2434


shuttle electrons between electrodes and analytes [47– 54]. In order to evaluate the electrocatalytic ability of the IrOx nanotube modified electrode toward oxidation of Cr3 + and As3 + cyclic voltammograms of the modified and unmodified electrodes were obtained in the presence and absence of the proposed analytes in the pH 5 buffer solution. Figure 7 shows cyclic voltammograms of modified and unmodified electrodes in the absence and presence of Cr3 + (100 mM) at pH 5 buffer solution. As shown for a bare glassy carbon electrode, no redox response can be seen for Cr3 + in the potential range from 0.0 to 0.8 V. However, at the IrOx-nanotube modified GC electrode, the oxidation peak current of the IrOx redox couple was greatly increased due to electrocatalytic activity of the modified electrode toward oxidation of Cr3 + . The decreased overvoltage and increased peak current of IrOx redox couple during chromium oxidation confirms that IrOx nanotubes have high catalytic ability for Cr3 + oxidation. Therefore, IrOx nanotubes are suitable as mediators to shuttle electrons between Cr3 + and working electrode and facilitate electrochemical regeneration following electron exchange with Cr3 + . For investigation of the electrocatalytic activity of the modified electrode toward arsenic oxidation the experiment was repeated in buffer solution containing 100 mM of As3 + . The modified electrode shows the same catalytic activity toward arsenic oxidation




(results not shown). In order to evaluate the electrocatalytic activity of cobalt oxide film, the cyclic voltammogram of the modified electrode in the presence of different concentrations of Cr3 + was recorded (Figure 8). As shown, the anodic peak current increased by raising the concentration of Cr3 + . Based on the results the following catalytic scheme (EC’) describes the reaction sequence of the oxidation of Cr3 + by IrOx redox center. Ir2 O3 xH2 OðaqÞ þ 2OH

ð4Þ

! 2IrO2 xH2 OðsÞ þ H2 O þ 2e 6IrO2 xH2 OðsÞ þ 2Cr3þ þ 3H2 O ! 2Cr6þ þ 3Ir2 O3 xH2 OðaqÞ þ 6OH

ð5Þ

The catalytic peak currents are proportional to the concentration of Cr3 + in the range from 30 mM to 90 mM according to the equation: Ip(mA) = 0.0217[Cr3+](mM)0.008 mA and R2 = 0.996. The detection limit is estimated to be 1.8 mM when the signal to noise ratio is 3. It can be inferred from these results that the presence of the electrodeposited IrOx nanotubes on the surface of GC electrode facilitates the detection of Cr3 + at a low concentration level. Since differential pulse voltammetry has higher sensitivity than cyclic voltammetry it was used to measure lower concentrations of Cr3 + . Differential pulse voltammograms of the IrOx nanotube modified electrode in the presence of different concentrations of Cr3 + were recorded (Figure 9A). As shown a well-defined differential pulse voltammogram was observed, whith peak currents that linearly increased with increasing Cr3 + concentration in the range from 1–10 mM. The

plot of peak current vs. Cr3 + concentration fitted the equation: Ip (nA) = 1[As+3] mM + 0.5 nA, (R = 0.9818). The detection limit is estimated to be 0.2 mM when the signal to noise ratio is 3. The differential pulse voltammograms of the modified electrode in solution containing different concentration of As + 3 is shown in Figure 9B. The Inset of Figure 8 shows that the peak current is proportional to the concentration of As + 3 in the range from 1–8 mM, Ip (nA) = 3.8 [As + 3] nM1 nA, (R2 = 0.9933). The detection limit is estimated to be 0.10 mM when the signal to noise ratio is 3. The determination of analytes in the presence of common interfering ions was investigated. Differential pulse voltammograms of the modified electrode in the presence 100 mM of interfering substances and 3 mM of Cr + 3 and As + 3 were recorded. The results indicate that no response is observed for the modified electrode in the presence of different interfering substances such as Cu2 + , Mn2 + , Cd2 + , Zn2 + , Ni2 + , Pb2 + , Co2 + , Sn2 + , Hg2 + and V3 + . At higher concentration ratios the interfering effect of Sn2 + is recognizable. Therefore, the modified electrode can be used for detection of Cr3 + and As3 + in the presence different reducing and oxidizing agents.

4 Conclusions In summary, a simple, fast and low cost electrochemical method was used for electrodeposition of IrOx nanotubes deposited into PC templates containing cylindrical nanopores with 100 nm diameter. SEM images indicated that the fabrication technique used in this study is well adapt-

Fig. 8. Cyclic voltammetry response of IrO2 nanotube modified electrode in phosphate buffer solution (pH 5) at scan rate 20 mV s1 in the presence of different Cr3 + concentrations, from inner to outer, 0.0, 30, 40, 50, 60,70 and 80 mM. Insets: plot of catalytic currents vs. Cr3 + concentration. Electroanalysis 2011, 23, No. 10, 2429 – 2437



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Fig. 9. Differential pulse voltammograms of IrO2 nanotube modified electrode in the presence of different concentrations of Cr3 + (A) and As3 + (B). Insets: plots of peak current vs. Cr3 + and As3 + concentrations.

ed to the synthesis of uniform IrOx nanotubes with diameters of 100–110 nm and lengths of 1–3 mM. The glassy carbon electrode modified with electrodeposited IrOx nanotubes showed well defined response due to the Ir(III)/Ir(IV) redox system. The modified electrode shows electrocatalytic activity toward oxidation of Cr3 + and As3 + . The modified electrode was used for micromolar or lower concentration detection of the selected analytes using differential pulse voltammetry. The relationship between current responses and Cr3 + and As3 + concentrations are linear up to 80 mM with detection limits of 0.2 and 0.1 mM, respectively. Our results demonstrate that cyclic voltammetry can be promised as simple and low cost method for fabrication of IrOx nanotubes, using polycarbonate template as nanopore membrane.

Acknowledgements This research was supported by the Iranian Nanotechnology Initiative and the Research Office of the University of Kurdistan. The authors wish to thank Dr. Richard Superfine, Briana Fiser (Department of Physic and Astronomy, University of North Carolina at Chapel Hill) and Dr. Ste2436


phane Xavier (Nanocarb laboratory, Thales polytechnique, Palaiseau, France) and specifically thanks to Dr. Vincent Callegari (Poly-unite de chimie et de physiqu des hauts polymers, Louvain, Belgium) for their valuable discussions.

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